A protection relay, also referred as Intelligent Electronic Device (IED), is a microcontroller based intelligent electronic device with a basic function to protect electrical equipment by tripping a circuit breaker and interrupting a power line in case of abnormal situations. The tripping signal for the trip coil or other actuator of the circuit breaker can be generated by the protection relay-when a particular parameter (e.g., line or phase current) exceeds a nominal or preset value for a predefined time period.
A self-supplied (self-powered) protection relay utilizes energy from the current sensing transformers to supply to the relay electronics circuit and provide the energy required to operate trip coils. The design of a self-supplied relay has several constraints associated with it to ensure the measurements are accurate and sensitive to avoid any mal-operation. Also, special provisions are required to make its circuitry efficient and optimized for power consumption. Some of these constraints and methods to generate a power supply by controlled charging for operation of the self-powered relay are disclosed in WIPO publications WO2009101463 and WO2012056277.
A self-powered relay requires a minimum time and phase currents to get powered on. Therefore, the measurement and protection functionalities can only be performed after the self-powered relay is powered on from the energy in the circuit involving the electrical equipment being protected. In conditions where the relay is required to switch on in a fault condition (i.e., there may be a high current because of the fault), it is desirable that the self-powered relay switches on as fast as possible and also performs detection of fault conditions at the earliest time to minimize any possibility of damage due to the fault. The delay contributed by switching on of the relay and to detect fault or normal conditions to decide on tripping gets reflected as a Switch On To Fault (SOTF) time which is a parameter used to evaluate speed of a protection system.
A state of the art protection relay working on microcontrollers with 75 MHZ operating frequency as an example is likely to consume around 50 milliseconds for the activities mainly involving power stabilization (powering up using the line current), power on reset delays (initialization) and non-volatile memory read operations. The controller electronics (base module) starts executing the protection algorithm only after around 50 milliseconds from the instant device is powered on. This duration is a crucial period for SOTF time.
A protection system with a lower SOTF time is expected to operate faster and is safer. The startup time of the processing device (e.g., microcontroller) used in self-powered relays is a major contributing factor to SOTF time. The startup includes time to initialize (boot) and the time to execute the protection algorithm used for the purpose of computing normal/abnormal condition. A reliable and less complex algorithm is certainly preferred to have the SOTF time and reliability within acceptable limits.
Further, the trip signal from a self-powered relay is provided with help of a trip capacitor that provides the power required to drive the coil. Therefore, in a self-powered relay, a fast trip capacitor charging circuit to have faster SOTF is desired and the charging method to be developed should consider challenges in a self-power device.